1. Field of the Invention
This invention relates to laser systems and in particular to a laser material processing system providing high resolution and power density of the laser beam.
2. Description of the Related Art
Laser technology has numerous applications in different scientific and commercial settings. For example, laser-based imaging and fabrication, such as engraving and cutting to form artwork or other ornamental images including lettering, has become very popular, especially in the creation of artwork on various materials, such as plastics, wood, rubber and rubber-like material, paper, etc.
As the state of the art of such laser technology and applications has progressed, one trend in the laser engraving field has been to produce finer and more precise detail in the engraved and/or cut images. Typically, commercially available laser beam delivery systems easily provide resolutions at which a laser beam may be positioned on the order of about 0.01 mm. using known X-Y based motion systems. An example of a known laser beam delivery system 10 is illustrated in FIG. 1, having a movable beam focusing assembly 34 which travels back and forth as shown by arrows 14 on a moveable arm 15 forming an X-axis, the arm 15 moving back and forth as shown by arrows 12 on stationary arms 13 and 16, which may be rails, forming a Y-axis and positioned over a workpiece 18 of material for engraving, etching, or cutting artwork or the like with a laser beam 20 output from a laser source 22.
The system 10 operates by directing the output laser beam 20 along paths 24–28 parallel to the Y-axis and the X-axis, respectively, using mirrors and/or other known optical elements. A final mirror 30 and a final focusing lens 32 are located in beam focusing assembly 34 of the system 10, such that the final mirror 30 directs the beam 20 out of the plane of the X-Y motion system to the final focusing lens 32. The final focusing lens 32 then focuses the resulting beam to a focal spot 36 on the subject material workpiece 18.
In an example embodiment, the laser source 22 is a typical carbon dioxide laser, providing an output beam between about 3 mm. and about 5 mm. in size, which may be focused to a focal spot typically no smaller than about 0.1 mm. at a typical focal length of about 50 mm., or about 10 times larger than the resolution at which known motion systems may position the focal spot 36. Such configurations of known laser systems generally limit the detail which may be achieved in the cut or engraved image on the workpiece 18.
One obvious solution to such limitations in detail is to focus the output beam 20 from the laser source 22 to a smaller spot. Traditionally, such focusing is done by employing a beam expander, using known optical elements, to expand the output beam from the laser source 22 by about 2 times to about 10 times. With D being the input beam diameter, the proposed solution increases D, which then allows the beam to be focused to a spot about 2 times to about 10 times smaller. Such focusing decreases the focal spot diameter d, and so such use of a beam expander provides smaller focal spot diameters than could be achieved with an unexpanded beam, as determined by the following equation:d(1/e^2)=4λf/(πD(1/e^2))where f is the focal length, λ is the laser wavelength, and d(1/e^2) and D(1/e^2) correspond to the focal spot diameter d and the input beam diameter D measured at the 1/e2 points of the laser beam output from the laser source.
In the prior art, a beam expander would typically be placed at a convenient location in the beam path close to the position where the beam 20 exits the laser source 22. However, such an implementation presents a problem in that an expanded beam with a larger diameter requires larger and often heavier optics to direct the expanded beam along the axes and arms of the motion system. In addition to increasing costs, such a need for larger and heavier optics has other undesirable effects on the design of the motion system, such as requiring larger clearances for the beam, as well as stronger and/or larger motors to move the larger optics and support structures of the motion system, which ultimately drives up the purchase and maintenance costs to implement the laser beam delivery system 10.